The present invention relates to a wheel for a rotating flow machine.
Although the invention is primarily intended to provide a fan wheel, turbine wheel or compressor wheel or the like for use in a gas turbine, such as an aircraft engine, preferably a jet engine, the wheel according to the invention could also be used in a stationary gas turbine for the generation of power or in other rotating flow machines of various kinds. Examples of other flow machines in which the invention could be used are steam turbines, pumps, fans, etc.
Fan wheels, turbine wheels and compressor wheels for gas turbines have a rotor and a plurality of blades that are arranged on the rotor and that extend radially out from the rotor. In order to increase the rigidity of such a wheel and to reduce the tendency to vibrate, according to previously-known technology, discrete structurally-supporting elements in the form of so-called dampers are arranged between the blades, preferably approximately centrally to the blades. On a turbine wheel, structurally-supporting elements or a continuous ring can be arranged at the tops of the blades to join the blades together. Such structurally-supporting elements and rings have, however, the disadvantage that aerodynamic losses arise, the weight of the wheel is increased and the centrifugal forces on the blades and the rotor increase.
A further disadvantage of the use of structurally-supporting elements is that the manufacture of such a wheel is made more complicated, as the structurally-supporting elements must be manufactured and matched to the blades and fitted on the wheel. The use of a continuous ring that is attached at the tops of the blades has the disadvantage that the ring and the blades must be joined together, for example by welding. On a fan wheel, a ring cannot be arranged at the tops of the blades as that would result in too large centrifugal forces when in operation. In addition, it is more difficult to repair the wheel when it is provided with structurally-supporting elements or a ring. In a wheel that does not have a fixed ring and that is provided with so-called removable blades, an individual blade can be removed for repair relatively easily. When the rotor and blades are manufactured as one piece, the fitting of structurally-supporting elements will be made more difficult or impossible, depending upon the profile of the blades.
An alternative to structurally-supporting elements is to arrange a self-supporting outer ring around the blades. By self-supporting ring is meant a ring that does not cause large outward-directed forces on the blades due to centrifugal forces, while at the same time the ring can provide increased stability to the relatively thin blades in the same way as the structurally-supporting elements described above. Such a design is more rigid than a corresponding design that does not have a ring and structurally-supporting elements, which in turn means that resonance frequencies can be avoided. A ring arranged at the tops of the blades results in a larger friction surface between the wheel and the stationary component which results in increased damping and less vibrations. In addition, the ring can be arranged in such a way that it loads the blades radially in the direction towards the rotor which reduces the centrifugal forces acting on the blade and the rotor originating from the blades. Additional advantages of such a ring are that leakage at the tops of the blades can be avoided and that the blades can be made thinner, which reduces losses and makes possible a higher mass flow through the gas turbine. These advantages make it easier to design the blades in an aerodynamic way and thereby improve the performance of, for example, an aircraft engine.
For aircraft engine applications, the ring has, in addition, advantages relating to fulfilling the so-called containment requirement that specifies that if fragments, such as a turbine blade or parts thereof, come loose from the rotor for any reason, such fragments must be retained inside the housing that surrounds the rotor in order to prevent the fragments from damaging other parts of the aircraft. By means of the ring, the force from such fragments can be distributed in an advantageous way and the fragments can be prevented from penetrating the surrounding housing in a radial direction. In addition, the use of a ring means that in the event of a collision with a bird, the force from such a collision will be distributed over a larger number of blades, so that the blade is not brought into contact with adjacent stators as a result of deformation.
A self-supporting ring for rotating units in gas turbine engines is described in U.S. Pat. No. 6,223,524. A rotating unit has a rotor provided with a plurality of blades that extend radially from the rotor and a ring that is arranged around the blades. The ring and the blades are attached with a push fit between the ring and the blades in a radial direction when the rotor is not rotating. The push fit is of the order of 0.01-0.1″ and is intended to provide a maximal engagement between the blades and the ring when mounting the ring on the rotor, that is in the stationary state when the rotor is not rotating, and a minimum permitted engagement between the blades and the ring during operation. The abovementioned disadvantages associated with manufacture and repair also apply, however, to a certain extent for a rotating unit that has a self-supporting ring of the type that is described in U.S. Pat. No. 6,223,524. Such a design makes high demands of precision in the manufacture of the individual components and also when assembling these into a unit in order to obtain the required push fit between the blades and the ring. In addition, maintenance and repair of the blades is made more difficult as a result of the fact that, when dismantling and subsequently refitting the ring, the push fit must be eliminated and subsequently recreated, and contact surfaces must be inspected.
It is desirable to provide a wheel for a rotating flow machine, preferably a gas turbine, of the type defined in the introduction, that can combine the advantages of a self-supporting ring while, at the same time, the wheel is relatively simple to manufacture and repair.
With the use of a ring arranged concentrically in relation to the rotor outside the tops of the blades so that the ring and the blades overlap each other when the ring and the blades are viewed in a radial direction, which ring is arranged to be caused to rotate by the blades when the rotor rotates, it is thus the case that no clearance is required between the tops of the blades and a stationary component in order to permit rotation of the wheel. Instead, the outside of the ring will rotate in relation to a stationary component, and the requisite gap between the outside of the ring and such a stationary component, which gap is arranged in order to permit a certain radial expansion of the wheel during operation, can be reduced considerably in comparison to the clearance that is otherwise required between the tops of the blades and the stationary component. The seal between the ring and stationary component can be made more efficient in comparison to the case when the tops of the blades are to form a seal against a stationary component. By means, for example, of a comb seal, the clearance can be reduced and also the geometry of the seal can be improved.
As a result of the extent of the blades in a radial direction and the internal dimension of the ring being matched to each other in such a way that there is a clearance between the tip of each blade and the ring in a radial direction when the rotor is not rotating, the rotor and blades on one hand and the ring on the other hand will be components that can be separated when in an unloaded state. Thus, the ring can be both assembled and dismantled, if necessary, in a relatively simple way.
By the use of the ring according to the invention, it should also be easier to design the wheel so that required forces between the blades and the ring are obtained in the hot state during operation in comparison to a ring that has a gripping fit on the blades as soon as it is assembled. By the selection of the size of the initial clearance, the gripping force during operation, and in particular during maximum speed of revolution of the wheel, can be controlled to be a required value.
In addition, the invention relates to an aircraft engine comprising a wheel according to the invention.
Other advantageous characteristics and functions of various embodiments of the invention are apparent from the following description.